The goal of this work is to understand the replication cycle of murine leukemia viruses (MuLV's), the role of MuLV's in leukemogenesis, and the mechanism of Fv-1 restriction, a naturally occurring cellular system which specifically inhibits the replication of certain MuLV's. The studies use a number of cell culture techniques for quantitation of virus infectivity. Defective variants of MuLV have been isolated and characterized. These include variants obtained in vitro and a variant produced specifically by murine leukemia cells. The origin of this variant and its role in leukemogenesis are being investigated.